1. Field of the Invention
The present invention concerns a yttria-based refractory composition for use in producing slurries needed for producing ceramic molds for use in casting reactive metals.
2. Description of Prior Art
Aqueous suspensions of ceramic particles, such as yttrium oxide, zirconium oxide, Yttria-alumina-zirconia, alumina, and zircon are used industrially to form ceramic articles due to their suitability for use as structural materials at high temperatures. These refractory materials often are also used for casting super alloys and reactive metals.
An example of such a reactive metal is titanium. Titanium normally reacts with materials used to form the mould, such as oxides, thereby releasing oxygen and forming oxygen-enriched titanium. A suspension is a system in which typically solid particles are uniformly dispersed in a liquid such as water. Particles in the order of less than about 1 μm can be classified as colloidal particles and a suspension of such particles is referred to as a colloidal suspension. Such suspensions are used as ceramic slurries for different purposes, as mentioned above. Ceramics normally are at least partially soluble in water. Furthermore ceramics tend to hydrate, forming a bond with water. To what extent and how quickly ceramics dissolve or hydrate, varies. Moreover, colloidal particles of ceramics may agglomerate in water. The extent to which ceramics dissolve, hydrate or agglomerate in water based systems depends on many factors, including the nature of the ceramic powder, the oxidation state of the ceramic, the pH, the temperature of the system and the dispersants which are used.
A lot of methods are known in the art to stabilize colloidal suspensions i.e. preventing the suspensions from agglomerating, while simultaneously reducing the dissolution and hydration rates. For instance, three known mechanisms include electrostatic, steric and electrosteric mechanisms. These mechanisms are reviewed in detail by Cesarano and Aksay “Stability of Aqueous Alpha-Al2O3Suspensions with Poly-(methacrylic acid) Polyelectrolyte”, J. Am. Ceram. Soc. 71 p 250-255 (1988).
In the U.S. Pat. No. 5,624,604 to Yasrebi et al. it is told that besides colloidal dispersion, reducing the attack of water (i.e. hydration and/or solvation) on the ceramic particle also is an important consideration for making commercially suitable ceramic slurries. Ceramic materials normally react with water and either partially dissolve (referred to as dissolution or solvation) or form hydrates. The extent of dissolution or hydration varies among different ceramic materials. As ceramic materials dissolve, the dissolved species may substantially change the ionic strength of the solution and consequently agglomerate the particles. In the case of particle hydration, some ceramics form a hydroxide surface layer. However, attack by water also may proceed farther than the surface layer and may advance into the body of the particle. As a result, size, morphology and the crystal phase of the particles may change.
In many commercially important ceramics, such as alumina (Al2O3), zirconia (ZrO2), and zircon (ZrSiO4) to name a few, the dissolution rate and the extent to which dissolution proceeds is low enough so that it does not seem to interfere with their aqueous commercial use, at least under mild acidic or basic conditions such as from about pH 3 to about pH 11. Furthermore, hydration does not seem to form more than a thin surface layer, at least when the particle size is equal to or larger than one micrometer. However, other commercially important ceramics, such as magnesia (MgO), yttria-alumina-zirconia, and Y2O3 (yttria), dissolve in an aqueous media to much larger extent and at faster rates than the ceramic materials discussed above. As a result, aqueous processing of these materials such as magnesia, calcia, yttria, yttria-alumina-zirconia is either difficult or even not practicable. Many attempts have been made by persons skilled in the art of ceramic processing to reduce the dissolution and hydration of ceramic particles, while simultaneously keeping the ceramic particles dispersed (unagglomerated) in suspensions. For example, Horton's U.S. Pat. No. 4,947,927 teaches that by adjusting the pH of a yttria slurry to high pH values in excess of pH 11 one can make yttria intrinsically less soluble in water, thereby decreasing its sensitivity to water attack.
Compared to electrostatic stabilization, electrosteric stabilization provides a better method for simultaneously dispersing colloidal particles in suspension and reducing water attack on the ceramic surface.
The limitations of this method were presented by Nakagawa, M. Yasrebi, J. Liu and I. A. Aksay (“Stability and Aging of Aqueous MgO Suspensions”) at the annual meeting of the Am. Ceram. Soc. (1989). Also monomers have been used to prevent the agglomeration of alumina suspensions. Graule et al. “Stabilization of Alumina Dispersions with Carboxyclic Acids”. Proceedings of the Second European Ceramic Society Conference (1991).
U.S. Pat. No. 5,624,604 Yasrebi et al. teaches a method for dispersing and reducing the rate of dissolution and/or hydration of colloidal ceramic suspensions by adding a non polymeric hydroxylated organic compound to a ceramic suspension. The ceramic suspension typically comprises a colloidal suspension of a metal oxide wherein the metal of the metal oxide is an alkali metal, alkaline-earth metal or rare-earth metal but preferably is magnesium, calcium or a rare-earth metal.
Other methods for increasing the lifetime of a casting slurry are described in U.S. Pat. No. 6,390,179 by Yasrebi et al., thus one feature of the invention is processing refractory powders at a first hydration level to produce powders having a second, lower hydration level before the processed materials are used to form casting slurries. Processing according to the disclosed methods results in a substantial increase in the lifetime of a slurry made using such processed materials compared to slurries made using materials not processed as described herein.
U.S. Pat. No. 5,464,797 describes an aqueous ceramic slurry having from about 70-weight percent to about 85 weight percent of a fused yttria-zirconia material. The weight-percent of zirconia in the fused yttria-zirconia preferably varies from about 1.0 weight percent to about 10 weight percent. The slurries of the present invention are used to form ceramic mold facecoatings for casting reactive materials. These slurries are less sensitive to pH-fluctuations than slurries made from 100 percent yttria (yttria slurries).
Thus, it is understood that persons skilled in the art of ceramic processing have long searched for, and developed methods to increase the lifetime of casting slurries. Despite the prior inventions directed to this objective, there still is a need for convenient and practical methods for increasing the useful lifetimes of investment casting slurries in particular when using other (amongst others Ammonium Zirconium Carbonate, Zirconium Acetate), not colloidal silica based new binder systems to process such slurries.
In the U.S. Pat. No. 5,827,791 Pauliny et al focused yttria-based slurries for use in producing ceramic molds for use in the investment casting of reactive metals, particularly titanium and titanium alloys, where the specific preferred binders amongst colloidal silica are ammonium zirconium carbonate and zirconium acetate.
Remet Corporation, a leading company in providing binders for the Precision Investment Casting Industry, offers Ammonium Zirconium Carbonate (Ticoat®-N) and cites that it is an effective binder system specifically for titanium castings. Remet Corporation also offers Colloidal Zirconia, that is defined as an acetate stabilized binder for high temperature applications.
In the U.S. Pat. No. 4,740,246 Feagin focused relatively unreactive mold coatings with titanium and titanium alloys that are prepared from zirconia or yttria sols, or mixtures thereof as a binder for refractory such as zirconium oxide, yttrium oxide and mixtures thereof. Feagin cites an example, where a cast-sample was made of a slurry containing yttrium oxide and zirconium acetate as essential parts. This sample is very low in alpha case being less than 0.001 inch.
From U.S. Pat. No. 4,057,433 a mold for casting molten reactive metals is known, which has a facing portion comprising finely divided particles of the oxyfluorides of the metals of Group IIIa and a back-up portion comprising finely divided particles of shell mold back-up material.
The Institution of Electrical Engineers, Stevenage, GB; September 1979 (1970-09), Udalova L. V. et AL describe the compaction kinetics of Y2O3 doped with 0.4-3.0 wt % LiF at 20-1250° C. and a specific pressure of 1000 kg/cm2.
Takashima M. published in the Journal of Fluorine Chemistry; Elsevier Sequoia, Lausanne, CH, vol. 105, no. 2, September 2000, pages 249-256 an article about the “Preparation and properties of binary rare-earth oxide fluorides” which are obtained by the solid-solid reaction between rare-earth oxide and fluoride at a temperature higher than 1000° C.
The Institution of Electrical Engineers, Stevenage, GB; November 1980 (1980-11), Udalova L. V. et al; describe in the published article “General features of compaction of powders of certain Lithiumfluoride doped powders” the reaction of pure yttrium oxide powder with lithium fluoride powder upon compaction at high pressure at room temperature.